1. Field of the Invention
The present invention relates in general to electronic circuitry and in particular, to current mirroring circuitry and methods.
2. Description of the Related Art
Current mirrors have many applications, including those in analog circuits such as operational amplifiers, comparators, band gap references, and the like. In one common current mirroring technique, a reference current is generated and then replicated (mirrored) through parallel weighted current paths using a set of matched transistors of appropriate aspect ratios. The resulting set of graduated currents can then, for example, be used to bias or drive other circuits in the device in varying ratios.
One of the drawbacks of using parallel transistors in current mirrors is their sensitivity to such factors as power supply variation, operating temperature variation, and fabrication process tolerances. Temperature and power supply variation can cause the currents to drift from their nominal design values, unless suitable compensation is provided for making the current paths track with the temperature and power supply changes. Fabrication process variations can cause the conductance of the transistors, and hence the accuracy in the current mirroring, to vary from wafer to wafer or even between circuits on the same wafer. For example, even if the channel width to length ratios of the transistors are held well within design tolerances, other factors such as differences in oxide thickness, carrier mobility and carrier doping levels can still cause variation in transistor conductivity.
Given the usefulness of current mirrors in a wide range of electronic circuit applications, better techniques are needed for compensating for temperature, process, and/or power supply variation in circuits employing current mirrors.